1. Field of the Invention
This invention relates to audio processing.
2. Description of the Prior Art
In applications such as digital fingerprinting or watermarking (which may collectively be referred to by the term forensic marking), a payload signal may be inserted into a primary audio signal in the form of a noise pattern such as a pseudo-random noise signal. The aim is generally that the noise signal is near to imperceptible and, if it can be heard, is not subjectively disturbing. This type of technique allows various types of payload to be added in a way which need not alter the overall bandwidth, bitrate and format of the primary audio signal. The payload data can be recovered later by a correlation technique, which often still works even if the watermarked audio signal has been manipulated or damaged in various ways between watermark application and watermark recovery.
Examples of the type of payload data which can be added include security data (e.g. for identifying pirate or illegal copies), broadcast monitoring data and metadata describing the audio signal represented by the primary audio signal.
The noise signal can be modulated before being added to the primary audio signal. This means in general terms that the level of the noise signal is increased when the level of the primary audio signal increases, and is decreased when the level of the primary audio signal decreases. In this way, more of the payload data's noise signal (giving a potentially better recovery of the payload data) can be included when it can be masked by louder passages in the primary audio signal.
However, if the noise signal tracks the primary audio signal too closely it can become audible and potentially subjectively disturbing, especially with sounds such as drum beats and the like.
In envelope-controlled audio processing systems a time constant can be applied to the rise time and fall time of the controlled signal (in this example, the noise signal). These are known as the attack and decay (or release) time constants. If such measures are applied to the present example, the result is that a rapid rise in the primary audio signal level causes a slower rise in the noise signal. This is quite acceptable—even desirable in some circumstances. But it is more of a problem that a sudden decrease in the primary audio signal level would lead to a slower decrease in the noise signal level. In an extreme case this could lead to undesirable situation of the noise signal being instantaneously larger than the primary audio signal.